Custom fields are fields whose values are based on expressions.  Adding a new custom field can be done by clicking the "New Custom Field" button.  See below:

Field expessions are formulas according to which a custom field's value is determined. To set the field's expression, double-click on it.  This can also be achieved by right-clicking on a field and clicking on the Edit Expression button.

 

Field expressions are made up of a combination of functions, fields, operators and costants. 

Example:  [Profit] = [Sales] - [Cost]

The value of the field Profit is calculated by substracting the corresponding value in the [Cost] field from the value in the Sales field.

See Also